How do I properly set up Subsurface Scattering for a human skeleton for Cycles?
Under the green label on the right with Principled BSDF turn up Subsurface for subsurface scattering; also adjust the Subsurface color. You can also increase the Subsurface Radius very slightly.

According to the following answers: Principled BSDF subsurface scattering values for skin material the bones should probably have a subsurface value of less than 0.1. How accurate that looks mostly will depend on the lighting and your own ability to reference an image.

Because subsurface scattering varies depending on the distance from the edge of perceived light, along with the distance traveled through the mesh, I don't believe you need to map its value.